Hydrometallurgical copper mining operations commonly use a leaching system and a copper extraction plant, particularly a solvent extraction/electrowinning (SX/EW) plant, to recover copper. Currently, electrowon copper accounts for about 30% of total U.S. copper production. Worldwide, there are more than 26 major heap, dump, or in-situ leaching operations using SX/EW, with a total capacity of 800,000 tons of copper annually. Copper mining operations using leaching and SX/EW are able to process low-grade ores profitably due to low labor, capital, and operating costs.
In copper leaching, a lixiviant, typically aqueous sulfuric acid, is contacted with rock or ore containing the copper to solubilize the copper in the lixiviant and form a pregnant leach solution containing dissolved copper. The contact of the lixiviant and rock can be performed in a tank or other vessel (known as agitation or vat leaching) or on an impervious leach pad upon which the rock is formed into a pile or heap (known as heap leaching).
The steps required to extract the dissolved copper from the pregnant leach solution depend upon the selected recovery method. In an SX/EW plant, the pregnant leach solution is contacted with an organic collector, such as hydroxy phenyl oximes, typically at a pH ranging from about pH1 to about pH3 in a liquid, commonly referred to as the "lix," to cause the dissolved copper to attach to the organic collector to form a loaded organic, and the loaded organic collector is later contacted with an electrolyte or stripping solution of about 100-200 g acid/L to resolubilize the copper in a rich stripping solution. The barren raffinate is recycled to the leaching step, and the barren lix to the step of copper extraction from the pregnant leach solution. In an IX/EW plant, the pregnant leach solution is contacted with an ion exchange resin, typically at a pH ranging from about pH1 to about pH3, and the copper ions are transferred to the ion exchange resin. The copper-rich ion exchange resin is then contacted with the stripping solution of about 100-200 g acid/L to transfer the copper from the ion exchange resin to the stripping solution or electrolyte. In either case, the copper-rich electrolyte or stripping solution is introduced into an electrowinning cell where copper is recovered on an electrode and the barren electrolyte is subsequently recontacted with the copper-loaded organic solution.
Contaminants in the various process streams in the above-described process can reduce copper recovery. By way of example, organic collector that is carried over into the copper-rich or copper barren electrolyte streams (i.e., in the electrowinning circuit) and/or multi-valent metals can foul/contaminate the copper cathode in the electrowinning cell, reduce current efficiency and copper product quality, and cause poor copper removal from the cathode blank. Bleed streams have been used in the past to control the build-up of such contaminants. Bleed streams, however, require the replacement of large quantities of acid and clean water (which is costly) and remove a substantial amount of copper (and expensive cobalt additive) from the electrolyte circuit. Excess multivalent copper ions in the stripping circuit can also create problems because the driving force for solubilizing the copper attached to the organic collector or ion exchange resin is dependent directly on the copper concentration in the barren electrolyte. Suspended and colloidal solids can further detrimentally impact the phase separation of the rich electrolyte from the barren lix due to the formation of "crud" (an emulsion of organic collector, pregnant leach solution, and suspended and colloidal solids), and they can also plug ion exchange resin beds. As used herein, "suspended solids" refer to solids having a size above about 0.45 microns, and "colloidal solids" refer to solids having a size below about 0.45 microns. Accordingly, reducing the copper and colloidal solid concentrations in the barren electrolyte can significantly increase the amount of copper concentrated in the rich electrolyte after the stripping step. The build-up of multi-valent metals such as silica, aluminum, zinc, cadmium, iron, manganese, calcium, and magnesium, and metalloids/semi-metals, such as arsenic and selenium, in the leaching circuit can detrimentally affect the solubility of copper in the lixiviant and thereby decrease copper recovery. Organic collector in the leaching circuit can also represent a large economic loss and create numerous environmental problems as it "coats" or contaminates the ore heap.